The invention relates to a series resonant converter comprising a control circuit for controlling the output voltage of the converter.
Such converters are used, for example, in high-voltage generators for X-ray apparatus.
In such converters is usually included a control circuit for controlling the output voltage, to keep them at a constant value in a steady state condition. However, basically the starting behavior of the converter is problematic. For example, a briefest possible rise time and a smallest possible overshoot at the end of the starting phase is desirable for rapidly reaching the steady state condition of the converter. When used in X-ray apparatus, reaching the steady state condition in the fastest possible way is to be strived for so as to avoid detrimental undesired radiation doses for the respective patients.
From U.S. Pat. No. 5,107,412 is known a series resonant converter which is used in high-voltage generators for X-ray apparatus and by which the desired starting behavior is not yet reached. The converter includes a full-wave bridge comprising thyristors. A current detector is provided for detecting zero crossings of the current flowing through the resonant circuit elements and delivered by the full-wave bridge. An ignition pulse generator generates ignition pulses for igniting the thyristors in dependence on the zero crossings detected by the current detector. The thyristors are then controlled such that a thyristor is not ignited until the current flowing through another thyristor conducting until then has dropped to zero and, furthermore, the so-called recovery time has elapsed. A zero crossing signal is not generated until the respective current in reverse direction has dropped below a threshold value (is), that is, has exceeded this value. To ensure that the converter also works with very small currents in the reverse direction of the thyristors, thus when such a threshold value is not reached, there is proposed to generate an auxiliary zero crossing signal when no zero crossing signal occurring in normal operation is generated within a specific period of time after an ignition pulse. Furthermore, the described series resonant converter includes a controller for controlling the converter output voltage, which controller compares the actual value of the high voltage delivered by the converter to a reference value (nominal value) and delivers controller output signal in a manner not further explained, whose magnitude is determined by the difference between the actual high voltage produced by the converter and the reference value.
It is an object of the invention to provide a series resonant converter having an improved starting behavior.
The object is achieved in that the control circuit is provided for processing a first actual value, which depends on the respective converter output voltage, and for processing a second actual value, which depends on the respective current flowing through the series resonant circuit elements of the converter, and in that the control circuit is provided for delivering a correcting variable determining the scanning ratio of a pulse-width modulated voltage delivered to the series resonant circuit of the converter.
The converter according to the invention helps to achieve, via an additionally processed second actual value, a dynamic behavior in the starting phase of the converter, which behavior is improved compared to conventional convertersxe2x80x94more particularly a shorter rise time, a reduced peak overshoot, a shorter settling time and a larger robustness against tolerances of converter elements. Conventional series resonant converters only have controllers in which the difference is formed between the actual value of the output voltage and a nominal value of the output voltage. A normally used analog controllerxe2x80x94customarily an analog PI controllerxe2x80x94is used for forming a correcting variable in dependence on this difference.
In an embodiment of the invention, a digital control circuit includes a state space control. This enables to adapt the converter control circuit to modified frame conditions with little expenditure, which may be effected via software modifications when a digital signal processor is used.
The second actual value preferably represents scanning values of the current at its peaks flowing through the series resonant circuit elements of the converter. More particularly, an integration is provided of the current flowing through the series resonant circuit elements of the converter so as to form from the values thus determined the scanning values used for forming the second actual value.
In the fundamental embodiment for the digital control circuit there is provided that for forming the correcting variable both a first product of a factor and the first actual value and a second product of a second factor and the second actual value are subtracted from a value derived from the difference between the first actual value and a nominal value that represents the DC output voltage to be delivered.
In a variant of an embodiment of the digital control circuit there is provided that for forming the correcting variable a cascaded controller structure is provided, more particularly in that
in an outer control loop both a first product of a factor and the first actual value and a second product of a second factor and the second actual value are subtracted from a value derived from the difference between the first actual value and a nominal value that represents the DC output voltage to be delivered,
in an inner control loop, after the difference value produced by the outer control loop has been processed according to a limiting function, both a third product of a third factor and the first actual value and a fourth product of a fourth factor and the second actual value are subtracted from the value processed according to the limiting function.
By means of the additional inner control loop, a limitation of the current flowing through the resonant circuit elements of the converter is achieved.
For the case where there is a small load present on the converter output there is proposed to convert the second actual value in the outer control loop according to a non-linear function into a value that is used instead of the second product, because for this case the converter controller can no longer be adequately converted with an embodiment based on purely linear controller structures. More particularly, for most applications it is sufficient to use as a non-linear function a partially linear function in which for the case of normal operation with small loads, also a simple proportionality factor is used as an approximationxe2x80x94but which has a different value.
In a further embodiment of the invention the difference between the first actual value and a nominal value representing the DC output voltage to be delivered is summed while the sum values thus formed are weighted with a further factor. In this way stationary control deviations are counteracted.